The present invention relates to fiber optics, and, more particularly, to filters for fiber optic systems.
Filters for selecting a narrow frequency band from a relatively broad band of transmitted electro-magnetic radiation, "light" herein, have many diverse applications. For example, such filters can be used to provide narrower bands of light than might be otherwise available from a laser or other source. In one application, light so filtered can be reflected back to a laser source to tune the laser to a desired band.
Fiber optic filters can also be used to improve signal-to-noise ratios by isolating a signal carrying band from adjacent frequency bands. Also, selective filters can be used to demultiplex wavelength division multiplexed (WLDM) optical signals. A diverse array of other uses can be recognized by analogy to electronic systems.
Until recently, filters for fiber optics used bulk optic approaches in which light was removed from an optical fiber to be filtered externally. The filtered light could then be reintroduced into a fiber for subsequent transmission.
One disadvantage of the bulk optic approaches is the bulk of the external filter and required couplers. Another disadvantage relates to the necessarily tight mechanical tolerances and resultant vulnerability to vibration and environmental changes. The complexities and losses involved in the bulk optic approaches have inspired a search for an effective way to filter light "in-line", i.e., without removing it from the fiber. Devices which function by interacting directly with the evanescent fields of guided light in a single mode fiber are thus of considerable interest as elegant and compact alternatives to bulk optic devices.
An approach to "in-fiber" filtering, applicable to single-mode fibers is disclosed by W. V. Sorin and H.J. Shaw in "A Single-Mode Fiber Evanescent Grating Reflector", Journal of Lightwave Technology, Vol. LT-3, No. 5, Oct. 1985, pp. 1041-1042. This article disclosed the use of a metal diffraction grating disposed upon the side-polished portion of an optical fiber. This arrangement provides narrow band reflections of light transmitted from a laser source.
The evanescent trailing fields in the neighborhood of the fiber core can be reached using a well-known technique in which the fiber is stripped of its coating, embedded with epoxy in a groove cut in a glass block, ground down to the vicinity of its core and highly polished. This technique is described in a disclosure by R.A. Bergh, G. Kotler, and H.J Shaw, entitled "Single-Mode Fibre Optic Directional Coupler", Electronics Letters, Vol. 16, No. 7, Mar. 27, 1980, pp. 260-261. Interaction between the diffraction grating and the evanescent field is obtained by placing the metal surface of the grating in contact with the polished portion of the fiber. An index oil can be applied between fiber and grating to remove air gaps which would otherwise decrease the extent of the evanescent field.
Herein, a fiber prepared according to the above technique is referred to as a "side-polished fiber". The region so-polished is referred to as the "side-polished region" of the fiber.
The grating used by Sorin and Shaw includes a series of parallel ridges periodically disposed on a pitch of 0.278 microns (.mu.m). The grating for such a filter can be fabricated using well-known holographic techniques. The interference front generated by an intersecting pair of collimated laser beams can produce a series of generally parallel interference lines. These lines can be used to expose a photoresist coated substrate. The exposed substrate can be processed so that the interference pattern is represented as ridges on the finished grating.
While subsequent analysis has shown that many of the advantages sought from in-fiber filters are obtained by the device disclosed by Sorin and Shaw, it compares unfavorably with external filters in one important respect: heretofore, broadband tunable in-fiber filters have not been available. In contrast, external filters using bulk optical elements have been provided which are tunable over a broad band. Herein, "broad band" and "broadband" are used to refer to tuning ranges which are large relative to the bandwidth being tuned.
In one bulk optic reflective filter approaCh, light is removed from an optical fiber and collimated using an optical lens. The collimated light is then directed against a bulk optic diffraction grating. The diffraction grating diffracts the incident light so as to reflect a narrow band of wavelengths back along the direction of the incoming collimated beam. The reflected band is then coupled back into the fiber.
In this bulk optic approach, tuning is achieved by tilting the bulk optic grating, thereby changing the wavelength back towards the fiber. Rotating the grating changes its effective spatial periodicity in the direction of the incident light. The spatial periodicity determines the frequency band which is superimposed constructively back along the vector of incidence.
As indicated above, such bulk optic approaches are disadvantageous in requiring stringent mechanical tolerances since light must be coupled back into the fiber whose mode diameter can be less than 10 .mu.m. Furthermore, considerable space is required for the arrangement of the bulk optic components. Accordingly, tunable in-line filters are desired.
Heretofore, the tunability of in-line fiber optic filters has been very limited. The filter disclosed by Sorin and Shaw can be tuned by rotating the grating relative to the fiber. This increases the spatial periodicity of the grating in the direction of the fiber. However, as the grating is rotated, the orthogonality of the ridges to the direction of propagation is diminished so that the quality of the reflected signal is impaired. Thus, the tuning range is practically limit.RTM.d to a bandwidth comparable to the bandwidth of the reflected signal.
Two other approaches for tuning a grating filter are disclosed by C.A. Park et al., in "Single-Mode Behavior of a Multimode 1.55 .mu.m Laser with a Fibre Grating External Cavity", Electronics Letters, Vol. 22, No. 21, Oct. 9, 1986, pp. 1132-1133. The spatial periodicity of the grating can be increased by heating. However, the tuning was limited to 13 angstroms (.ANG.) relative to a transmitted wavelength of 15,620 .ANG.. The tuning range was just slightly larger than the reported 10 .ANG. reflection bandwidth. An even weaker tuning effect was achieved by varying the refractive index of oil placed between the cladding and the grating. In addition to the limited tuning ranges provided, a disadvantage of the temperature and oil approaches are the impracticality of varying these parameters, especially over the ranges required to obtain greater than narrow-band tuning.
Thus, the tuning range of disclosed in-line fiber optic filters has been limited to less than twice the reflection band of the filter of interest. However, in many applications a tuning range at least an order of magnitude greater than the reflection bandwidth is desired.
The quest for a broadband tunable in-line fiber optic filter faces two major challenges. The first is the determination of a structure that can provide the desired tuning function. The second is determining a method of manufacturing a device with the required structure, given the dimensions and precision required in fiber optical systems. Both these challenges are met as described below.